Reflective element for fiber optic sensor

ABSTRACT

A reflective element for directing an optical signal into a fiber optic sensor having an optical fiber includes a plane containing a sharply defined straight line that separates between a first area of low reflectivity and a second area of high reflectivity. The plane is disposed parallel to a free end surface of the optical fiber so that the free end surface intersects the line of the reflective element, whereby relative movement between the free end surface of the optical fiber and the line in response to a physical change sensed by the fiber optic sensor induces variations in an optical signal reflected by the reflective element through the optical fiber, which variations allow measurement of the physical change.

The present application is a continuation of U.S. patent applicationSer. No. 14/287,361, filed May 27, 2014, and entitled REFLECTIVE ELEMENTFOR FIBER OPTIC SENSOR, the entirety of which is incorporated herein byreference.

THE FIELD OF THE INVENTION

The present invention relates to fiber optic sensors, particularly tosensors substantially not affected by very strong electromagnetic fieldsable to work in high temperature conditions.

BACKGROUND OF THE INVENTION

Fiber optic sensors are known that use light energy and optical fibersto sense different physical parameters such as pressure, temperature,acceleration etc. Most of them consist of light source, photo detector,one or few optical fibers, reflective target and a sensitive to acertain physical effects element someway attached to the optical fibersor reflective target. Via a transmitting optical fiber light from alight source is dispatched to reflective target that partly reflects itback through a receiving optical fiber to a photo detector. Under acertain physical effects a sensitive element changes the relativeposition of the optical fiber and reflective target thereby changing theintensity of light reflected by the target into receiving optical fiberand transformed by the photo detector into electrical signal. Some ofthe fiber optic sensors include only one optical fiber combiningtransmitting and receiving fibers in one. Examples of such sensors aredisclosed in U.S. 2009/0123112, U.S. 2007/0247613 and U.S. Pat. No.5,771,091.

The reflective target is the most exacting and thus most expensive partof these sensors. Even small distortions of its shape or degradation ofits reflective surfaces caused by temperature variations candramatically deteriorate the sensor characteristics.

U.S. Pat. No. 4,915,882 discloses a method for manufacturing uniformlysmooth monocrystal reflectors of copper, silver or gold using a cruciblepolished to optical quality on the surface in contact with thereflecting surface of the monocrystal. It is noted that monocrystalreflectors withstand much better the extreme thermal loads caused bylaser beams, but that the advantages inherent in the monocrystallinestructure of the reflecting metal body are partially lost again duringforming and/or machining as these operations modify the homogeneouscrystalline texture. Reflection produced by monocrystal reflectors wasfound to be better for etched surfaces than in polished surfaces. Etchedsurfaces, however, are nonhomogeneous so that while of interest asprotective shields against laser beams they do not lend themselves tooptical or similar purposes in which a specific optical path requires aprecisely defined reflecting surface.

U.S. Pat. No. 4,414,471 discloses sensing of acoustic waves achieved byproviding spaced apart stationary and cantilevered optic fibers wherebyinertial forces created by acoustic signals modulate an optical signalcarried by the fibers through vibration of the cantilevered fiber. Inone embodiment, the sensor includes a cantilevered beam mounted at thefar end to a rigid structure and having a reflective member such as aconcave minor at the free end thereof. The end of optical fiber isdisposed at the center of the sphere of which the minor surface is asection. Light fed into the fiber is reflected from the minor, receivedby the fiber and applied to a detector at. When acoustic waves areincident on the transducer they will cause vibration of the cantileveredbeam due to inertial forces. The minor attached to the beam alsovibrates and amplitude modulates the light received by the mirror andreturned to the fiber.

Our co-pending U.S. Ser. No. 13/935,955, whose contents are whollyincorporated herein by reference, discloses a fiber optic accelerometercomprising a cantilever section which moves upon vibration oracceleration of the accelerometer such that its position relative to areflective target changes thereby reducing the instantaneous intensityof light reflected by the target into the second end of the opticalfiber and measured by the photo detector. The reflective target isformed of an optical fiber stub having a first end proximate the freesecond end of the optical fiber and a second end remote therefrom.

In one embodiment, the first end of the optical fiber stub has a slantedsurface formed at an angle to an optical axis of the optical fiber stuband the second end of the optical fiber stub is cut perpendicularly tothe optical axis and is coated with a highly polished efficient lightreflecting material.

In another embodiment, the first end of the optical fiber stub has astepped cut so as to present a first surface portion closer to the endof the optical fiber and a more distant second surface section and thesecond end of the optical fiber stub is cut perpendicularly to saidoptical axis and is coated with a highly polished efficient lightreflecting material.

In both cases, there is no change in reflectivity of the optical fiberstub, the variation in signal injected into the optical fiber beingcaused solely by the off-axis reflection of light from the optical fiberstub owing to the deflection of the cantilever such that movement of thefree end of the optical fiber causes a lessor or greater amount of thereflected light to be captured by the free end of the optical fiber. Thesame is true in U.S. Pat. No. 4,414,471 where the concave mirrorreflects light into the cantilever regardless of its deflection, thevibration of the minor serving to modulate the light prior to itsreflection into the free end of the fiber.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a reflective elementfor fiber optic sensors possessing in some cases sensitivity to acertain physical effects and having a simpler construction, and beinglow cost for its production.

This object is realized in accordance with the invention by a reflectiveelement for a fiber optic sensor having the features of the respectiveindependent claims.

The invention provides a reflective element for a fiber optic sensorbased on a single optical fiber, said reflective element comprising aplane containing a sharply defined straight line that separates betweena first area of low reflectivity and a second area of high reflectivity,said plane being disposed parallel to a free end surface of the opticalfiber so that said free end surface intersects said line, wherebyrelative movement between the free end surface of the optical fiber andthe line in response to a physical change sensed by the fiber opticsensor induces variations in an optical signal reflected by thereflective element through the optical fiber, said variations allowingmeasurement of the physical change.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 a shows schematically the construction of a reflective elementfor a digital fiber optic sensor made of mono-crystal material bytechnology of anisotropic etching and vapor deposition;

FIGS. 1 b and 1 c are graphical representations showing intensity of aseries of optical pulses directed to the free end of the fiber opticsensor of FIG. 1 a;

FIGS. 2 a and 2 b show schematically a respective cross-section and endelevation of a reflective element for a single axis fiber optic sensorbased on a single fiber;

FIG. 3 is a schematic view of a dual axis fiber optic sensor having twoindependent fibers and a single reflective element similar to thatdepicted in FIG. 2;

FIGS. 4 a and 4 b show schematically a reflective element for a fiberoptic pressure sensor in an initial and subsequent deflected state,respectively;

FIG. 5 shows a schematic view of a reflective element for a fiberdynamometer made of mono crystal material; and

FIG. 6 shows a schematic view of a reflective element for a fiber optictemperature sensor made of mono crystal material.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description of some embodiments, identical componentsthat appear in more than one figure or that share similar functionalitywill be referenced by identical reference symbols.

FIG. 1 a is a schematic cross-section showing construction of areflective element for a fiber optic sensor. The reflective element maybe a plate made of mono-crystal material 1 with multiple parallelreflective stripes 2 disposed on the side faced to the free end 3 of anoptical fiber 4 and separated by v-grooves. The v-grooves define linesseparating the areas of high reflectivity from the areas of lowreflectivity. The optical fiber 4 emits light 5 toward the reflectiveelement and collects the back reflected light 6. Each reflective stripe2 comprises an uppermost reflective surface 7 deposited on a substratelayer 8 by vapor deposition or sputtering. The reflective surface 7 maybe formed of a material having high reflectivity such as gold while thesubstrate layer 8 may be formed of a material having medium reflectivitysuch as platinum or nickel such that the respective reflectivity of thetwo layers is different. The v-grooves are made by wet anisotropicetching. The angle 8 between the opposing faces of the v-grooves dependsof the selected mono-crystal material and should be no greater than 70°.In this case the intensity of light reflected in the direction of thefree end of the optical fiber will be close to zero. Thus when, under agiven physical load, the optical fiber 4 is displaced in the direction10 relative to the reflective element and its free end 3 intersects thelines between adjacent areas of high and low reflectivity, the intensityof the reflected light collected by the free end 3 of the optical fiber4 appears in the time domain 12 as a series of pulses having anasymmetrical profile 13 as shown in FIG. 1 b. When the optical fiber 4is displaced in the reverse direction 11, the profile of the pulses inthe time domain will be of inverse shape as shown in FIG. 1 c. So thenumber of pulses and the shape of their profile define the magnitude anddirection of optical fiber displacement. Such a reflective element maybe used for direct digitization of a fiber optic sensor output signal.

FIG. 2 a shows schematically a partial cross-sectional view of areflective element 14 for a single axis fiber optic sensor based on oneoptical fiber 4 wherein the reflective element has the shape of flathollow frame made of mono-crystalline material 15 with one face 16coated with a highly polished, efficient light reflecting material suchas gold so as to form a good reflecting surface 7. The internal edges ofthe aperture are beveled to form a truncated square pyramidal shapedaperture 17 with the base of the pyramid remote from the reflectiveface. The aperture is preferably formed by means of anisotropic wetetching, whereby the internal edges 18 and 19 are formed absolutelystraight and strictly orthogonal to each other. The classic approach isby etching a hole in <100> silicon wafer using a chemical such aspotassium hydroxide. The result is a square pyramidal shape aperture.The selected reflective material may be deposited by vapor depositiontechnology. Both of these technologies are well-suited to massproduction enabling the manufacturing price of so delicate and precise acomponent to be dramatically reduced. The free end 3 of the opticalfiber 4 emits light 5 in the direction of the reflective element andcollects the reflected light 6. Only movement of the free fiber end 3 ina direction perpendicular to the edge 19 shown in FIG. 2 b can producevariation of intensity of the reflected light and thus the valuablesignal while movement in the parallel direction to the edge 19 cannot.Thus the reflective element produces a spatial filtration of a fibermovement making the fiber optic sensor sensitive to a given physicaleffect such as acceleration, transverse force, deformation, etc. in onlyone direction.

FIG. 3 is a schematic view of a dual axis fiber optic sensor employingthe reflective element 14 depicted in FIG. 2 in combination with twoindependent optical fibers 21 and 27 capable of displacement in randomdirections under a given physical load. The free ends of both fibers arerespectively mounted proximate the internal orthogonal edges 18 and 19of the reflective element 14. Thus the reflective element produces aspatial filtration of movement of the two fibers simultaneously makingthe fiber optic sensor sensitive to physical effects such asacceleration, transverse force, deformation, etc. in two strictlyorthogonal directions as shown by the arrows 28, 29.

In the embodiments described so far, the sensor signal is obtained uponmovement of the optical fiber in a direction that is perpendicular to afixed edge of the reflective element. Only the optical fiber moves withthe surface of its free end being substantially parallel to the highlyreflective surface of the reflective element, which does not move.However, the equivalent effect can be achieved using otherconfigurations wherein the reflective element itself moves in responseto an applied force. In some embodiments movement of the reflectiveelement induces movement of the free end of the optical fiber, whilestill retaining some relative movement with an edge of the reflectiveelement. In other embodiments, the free end of the optical fiber remainsfixed in space so that the required relative movement with an edge ofthe reflective element is induced by motion of the reflective elementonly. Non-limiting examples of these embodiments will now be described.

FIG. 4 a shows a schematic cross section of a fiber optic pressuresensor 30 having a sensor housing 31 supporting the optical fiber 4 andwherein a reflective element is formed as a generally L-shaped diaphragm32 sealing a channel 33 in the sensor housing 31. The diaphragm 32 isformed by wet processing of mono crystal material and has a generallyelongate body portion that spans the width of the channel 33 terminatingat an end of the channel proximate the free end of the optical fiber 4in a stepped portion 34 whose height is about half the diameter of theoptical fiber 4. The internal face of the stepped portion 34 is coatedwith a highly polished efficient light reflecting material so as to forma reflective surface 7. The optical fiber represents a cantilever beamthat passes beneath a membrane parallel to its plane and under itscenter on the minimal distance from its inner surface. The optical fiber4 conveys the light from a source of light (not shown) to the reflectivesurface 7 and conveys the reflected light back to a photodetector (notshown). Under applied force, P, the diaphragm is deflected by adeflection 37 as a function of applied force thereby applying a bendingforce to the optical fiber 4 and changing the position of its free endrelative to the reflective stepped portion 34 (FIG. 4 b). Consequently,the intensity of light conveyed to the photodetector will also changeproportional to the applied force, P.

FIG. 5 shows a schematic cross section of a reflective element in theshape of cantilever beam 38 for a fiber optic dynamometer wherein thebeam 38 is made of mono crystal material by wet etching with a steppedportion 34 on its free end coated by a highly reflective material toform a reflecting surface 7. Under applied force 39 the beam 38 bendsand thus changes the position of the reflective stepped portion 34relative to the free end 3 of the optical fiber 4. Consequently, theintensity of light conveyed to photodetector will also changeproportional to the applied force. The cantilever beam defines anelongated surface that is perpendicular to an internal face of thestepped portion 34 and to which an applied force induces deflection ofthe stepped portion 34 relative to the free end 3 of the optical fiber4.

FIG. 6 shows a schematic cross section of reflective element in theshape of cantilever beam 40 for a fiber optic thermometer wherein thebeam 40 is made of mono crystal material by wet etching with a steppedportion 34 on its free end coated by a highly reflective material toform a reflecting surface 7. One side of the beam 40 is coated with alayer of material 41 characterized by a coefficient of thermal expansion(CTE) that is very different from that of the mono crystal material ofwhich the beam 40 is formed. At ambient temperature variations, the beam40 bends and thus changes the position of the reflective stepped portion34 relative to the free end 3 of the optical fiber 4. Consequently, theintensity of light conveyed to photodetector will also changeproportional to ambient temperature change.

In the embodiment of FIG. 6, the cantilever beam defines an elongatedsurface that is perpendicular to an internal face of the stepped portion34 and that supports the layer of material 41. Typically, the layer ofmaterial 41 is coated on the elongated surface of the beam. But it couldequally well be riveted or attached thereto using adhesive as in knownper se.

1. A reflective element for a fiber optic sensor based on a singleoptical fiber, said reflective element comprising a plane containing asharply defined straight line that separates between a first area of lowreflectivity and a second area of high reflectivity, said plane beingdisposed parallel to a free end surface of the optical fiber so thatsaid free end surface intersects said edge, whereby relative movementbetween the free end surface of the optical fiber and the edge inresponse to a physical change sensed by the fiber optic sensor inducesvariations in an optical signal reflected by the reflective elementthrough the optical fiber, said variations allowing measurement of thephysical change.
 2. The reflective element according to claim 1,comprising multiple abutting areas of respective high and lowreflectivity each separated by respective sharply defined straight edgesthat are intermittently intersected by the free end surface of theoptical fiber in response to said physical change.
 3. The reflectiveelement according to claim 2, wherein the reflective element comprises aplate supporting multiple parallel reflective stripes of highreflectivity and intermediate v-grooves of low reflectivity.
 4. Thereflective element according to claim 3, wherein the reflective stripesare deposited on a substrate layer of low reflectivity using vapordeposition technology.
 5. The reflective element according to claim 3,wherein the stripe is gold and the substrate is platinum or nickel. 6.The reflective element according to claim 3, wherein the v-grooves areformed by wet anisotropic etching.
 7. The reflective element accordingto claim 3, wherein an internal angle between opposing faces of thev-grooves is no greater than 70°.
 8. The reflective element according toclaim 1, being formed of mono-crystal material.
 9. The reflectiveelement according to claim 3, wherein the reflective stripes are metallayers of submicron thickness made by vapor deposition or sputtering.10. The reflective element according to claim 1, wherein: said planeincludes an aperture having a shape of a truncated square pyramid, abase of which is behind the plane; and at least one edge of the aperturefunctions as said sharply defined straight line that separates betweenthe first area of low reflectivity constituted by the aperture and thesecond area of high reflectivity constituted by the plane surroundingthe aperture.
 11. The reflective element according to claim 10, beingformed of mono-crystal material.
 12. The reflective element according toclaim 10, wherein the aperture is formed by wet anisotropic etching. 13.The reflective element according to claim 10, wherein the second area ofhigh reflectivity is formed of metal layers of submicron thickness madeby vapor deposition or sputtering.
 14. The reflective element accordingto claim 10, wherein two mutually perpendicular edges of said apertureare configured for disposing in spaced relationship with respectiveoptical fibers configured for independent displacement in randomdirections under an applied physical force.
 15. The reflective elementaccording to claim 1, comprising a generally L-shaped diaphragm havingan elongate body portion and a stepped portion that defines an internalface that is coated with a highly polished efficient light reflectingmaterial so as to form a reflective surface.
 16. The reflective elementaccording to claim 15, being formed of mono-crystal material.
 17. Thereflective element according to claim 16, being formed by anisotropicwet etching.
 18. The reflective element according to claim 15, wherein aheight of the internal face of the step is approximately half a diameterof the optical fiber.
 19. The reflective element according to claim 1,being a cantilever beam formed of mono-crystal material and having on afree end thereof a step having an internal face coated by a highlyreflective material that at least partially intersects the free endsurface of the optical fiber.
 20. The reflective element according toclaim 19, being formed by anisotropic wet etching.
 21. The reflectiveelement according to claim 19, wherein a height of the internal face ofthe step is approximately half a diameter of the optical fiber.
 22. Thereflective element according to claim 1, being a cantilever beam havingon a free end thereof a step having an internal face coated by a highlyreflective material that at least partially intersects the free endsurface of the optical fiber wherein an elongated surface of thecantilever beam perpendicular to said internal face supports a layer ofmaterial having a different coefficient of thermal expansion to that ofthe cantilever beam, whereby a variation in ambient temperature inducesthe beam to bend and thus moves the step relative to the free end of theoptical fiber.
 23. The reflective element according to claim 22, beingformed of mono-crystal material.
 24. The reflective element according toclaim 23, being formed by anisotropic wet etching.
 25. The reflectiveelement according to claim 22, wherein a height of the internal face ofthe step is approximately half a diameter of the optical fiber.